There is some evidence that the deformities may be associated with the structure of the container. In the experiments reported here, one-year-old bare-root cuttings of small-leaved lime (Tilia cordata) and field elm (Ulmus minor) were planted in a peat and pumice substrate in one-litre black plastic containers.
One container type was square and smooth-sided, another was square but with two vertical ribs on each inside wall, while the third was circular with inward-pointing closed cones and outward-pointing open-ended cones in its wall (Superoots(TM) Air-cell(R)). The plants were raised under anti-hail nets in Como, Italy. One year later, they were repotted in similar but larger three-litre pots. No root pruning was allowed.
At the end of each growing season, the proportion of deformed roots was much higher in the smooth-sided containers. Evidently, the production of deformed roots was encouraged by the smooth inner surfaces. It seems sensible, therefore, to avoid using smooth-sided containers in the early stages of producing young trees.
Effect of Container Design on Plant Growth and Root Deformation of Littleleaf Linden and Field Elm by Amoroso, Frangi, Piatti, Ferrini, Fini and Faoro (2010). HortScience 45 (12): 1824-1829. ISHS members can view HortScience from the website at www.ishs.org